feat: added coerce option, for providing specialized argument parsing

[bcoe]

No description provided.

[nexdrew]

Since a callback is used, I would expect yargs-parser to support asynchronous coercion, but it doesn't (yargs-parser is fully synchronous). Values given to the callback asynchronously are ignored.

Perhaps coerce functions should either return a value or throw an error to make it clear that coercion must be synchronous.

[nexdrew]

See #47